The present invention is generally related to control and operation of power converter devices, and, more particularly, to circuits and techniques that improve the startup performance of power converters using synchronous rectifiers.
DC/DC power converter devices are widely used in numerous applications, such as telecommunications and networking applications. A dc/dc converter is an electronics device that converts a raw dc (direct current) voltage input, usually with a certain variation range, to a dc voltage output that meets a set of specifications. With fast-growing technologies used in telecommunications equipment, the demands on the power density and conversion efficiency of dc/dc converters continue to increase. The converter typically includes a transformer, having primary and secondary windings wound around a common magnetic core. By opening and closing the main power switches for appropriate intervals, control over the energy transfer between the input and output is accomplished. The transformer provides an alternating voltage whose amplitude can be adjusted by changing the number of turns of the primary and secondary windings. Moreover, the transformer provides DC isolation between the input and the output of the converter. However, a transformer is not required in a non-isolated converter.
One of the most common DC/DC converter topologies is the forward converter. When the primary winding of the forward converter is energized by closing the primary switch, energy is immediately transferred to the secondary winding. Synchronous rectifier circuits are used in forward converters, as well as in flyback converters, buck converters, push-pull converters, and half-bridge converters, among others. In switching power supply circuits employing synchronous rectifiers, the diodes are replaced by power transistors to obtain a lower on-state voltage drop. The synchronous rectifier generally uses n-channel MOSFETs rather than diodes to avoid the turn on voltage drop of diodes that can be significant for low output voltage power supplies. The transistors are biased to conduct from source-to-drain (for an n-channel power MOSFET) when a diode would have been conducting from anode to cathode, and conversely, are gated to block voltage from drain-to-source when a diode would have been blocking from cathode to anode. Although MOSFETs usually serve this purpose, bipolar transistors and other semiconductor switches as also suitable.
In these synchronous rectifier circuits, the gate signals can be self-driven, i.e., the gate signal can be tied directly to the power circuit, or controlled-driven, i.e., the gate signal is derived from some point in the circuit and goes through some processing circuit before being fed to the MOSFET gate driver. In a power converter, the synchronous rectifier which conducts during the non-conducting period of the main power switch(switches) is called a freewheeling synchronous rectifier. The gate drive signal to a freewheeling synchronous rectifier plays a very important role in the startup process of a converter.
FIG. 1 shows conventional synchronous rectifiers in a forward converter 10. In this example, a DC voltage input Vin is connected to the primary winding of the power transformer by a MOSFET power switch Q1. A clamp circuit arrangement is also provided to limit the reset voltage. The MOSFET power switch Q1 is shunted by a series connection of a clamp capacitor Creset and a MOSFET switch device Q2. The conducting intervals of Q1 and Q2 are mutually exclusive. The voltage inertia of the capacitor Creset limits the amplitude of the reset voltage appearing across the magnetizing inductance during the non-conducting interval of MOSFET power switch Q1.
The secondary winding is connected to an output lead through a synchronous rectifier including MOSFET rectifying devices SR1 and SR2. Each rectifying device includes a body diode. With the power switch Q1 conducting, the input voltage is applied across the primary winding. The secondary winding is oriented in polarity to respond to the primary voltage with a current flow through inductor Lo, the load connected to the output lead and back through the MOSFET rectifier device SR1 to the secondary winding. Continuity of the current flow in the inductor Lo when the power switch Q1 is non-conducting is maintained by the current path provided by the conduction of the MOSFET rectifier device SR2. An output filter capacitor Co shunts the output of the converter.
Conductivity of the two rectifier devices SR1 and SR2 is controlled by the gate drive signals provided by the primary PWM (pulse-width modulated) control of switch Q1. The control signal to SR1 and SR2 can be derived from various ways, such as signals coupled from the power transformer T or other mechanisms that carry the primary PWM timing information. PWM includes, for example, an oscillator, a comparator, and a flip-flop. The output of the PWM provides a PWM drive signal.
In order to prevent transformer saturation and excessive heating or failure of the switch Q1 during startup, the PWM drive signal on the primary switch Q1 usually goes through a soft-start process. During a soft-start, the pulse-width of the gate signal to Q1 gradually increases from a very small duty-ratio to its steady-state duty-ratio. Since the drive signal of the freewheeling synchronous rectifier SR2 is by and large complementary to that of primary switch Q1, its duty ratio starts high and gradually reduces over the soft-start process. Consequently, during a startup, especially if the output has a pre-existing voltage (pre-bias) (which could be from other power sources in the system) before the converter starts, the large duty-ratio of SR2 will build a negative current in the output inductor Lo, which may cause the output voltage to drop, and further resulting in disturbance to other voltages that are coupled to this output. This output voltage drop and the disturbance to other voltages may be unacceptable to some of the loads connected to these voltages.
Therefore, it would be desirable to control the drive signal to the rectifier device SR2 during this startup process to address the above problem.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a circuit for controlling switching circuitry in a power converter device during a soft-start process wherein the pulse-width of a gate drive signal to a main switch of the power converter device gradually increases from a minimum duty-ratio to a steady-state duty-ratio. The circuit includes synchronous rectifier control circuitry adapted to gradually apply a gate drive signal to a freewheeling synchronous rectifier of the switching circuitry during the soft-start process by either controlling the amplitude or pulse-width of the gate drive signal to the freewheeling synchronous rectifier.
The synchronous rectifier control circuitry includes gate clamping circuitry. In one aspect of the invention, the gate clamping circuitry includes a diode series coupled to a resistor paralleled to a capacitor to provide voltage clamping of the gate drive signal applied to the freewheeling synchronous rectifier. In another aspect of the invention, the gate clamping circuitry comprises a transistor series coupled to a resistor paralleled to a capacitor to provide voltage clamping of the gate drive signal applied to the freewheeling synchronous rectifier. A diode is optionally series coupled with an emitter or base of the transistor to block the voltage when the freewheeling rectifier is gated low. The common of the gate clamping circuitry may also be coupled to a negative voltage potential. Resistors and capacitors may be provided in the main current path of the gate drive circuit of the synchronous rectifier.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a method for controlling switching circuitry in a power converter device during a soft-start process wherein the pulse-width of a gate drive signal to a main switch of the power converter device gradually increases from a minimum duty-ratio to a steady-state duty-ratio. The method provides for gradually applying a gate drive signal derived from the main switch of the power converter device to a freewheeling synchronous rectifier of the switching circuitry during the soft-start process by gradually releasing either the amplitude or the pulse-width of the gate drive signal to the freewheeling synchronous rectifier. In an aspect of the invention, the gate drive signal is altered by gate clamping circuitry.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a power converter device including a primary side and a secondary side electromagnetically coupled to one another through a transformer, the power converter including switching circuitry, soft-start circuitry, and control circuitry. Specifically, the switching circuitry is coupled to the transformer on the secondary side, wherein the switching circuitry includes a first synchronous rectifier device that conducts during the conducting state of the main power switch and a second freewheeling rectifier device that conducts during the non-conducting state of the main power switch; the soft-start circuitry is used to gradually increase the pulse-width of the gate drive signal to the main power switch from a minimum duty-ratio to a steady-state duty-ratio during start-up; and control circuitry is coupled to the gate terminal of the second freewheeling rectifier device to alter the gate signal applied thereto during start-up to modify the time in which the second freewheeling rectifier device conducts during the non-conducting state of the main power switch.
The control circuitry is adapted to gradually release either the amplitude or the pulse-width of the gate drive signal to the freewheeling synchronous rectifier. In an aspect of the invention, the control circuitry comprises gate clamping circuitry such as a diode or a transistor series coupled to a resistor paralleled to a capacitor to provide voltage clamping of the gate drive signal applied to the freewheeling synchronous rectifier. The transistor may further include a diode series coupled with an emitter or base of the transistor to block the voltage when the freewheeling rectifier is gated low. Optionally, the common of the gate clamping circuitry is coupled to a negative voltage potential.